Ferromagnetic powders which have been employed for a magnetic recording medium include maghemite, cobalt-doped maghemite, magnetite, cobalt-doped magnetite, berthollide of maghemite and magnetite, cobalt-doped berthollide of maghemite and magnetite and chromium dioxide. However, magnetic properties of these powders such as coercivity (H.sub.c) or maximum residual magnetic flux density (B.sub.r) are insufficient for making a so-called high density recording, and are not suitable for recording a magnetic signal having a short recording wavelength (not longer than about 1 .mu.m) or for making a tape having a narrow track width (not more than about 50 .mu.m). A great deal of research and development of ferromagnetic powders having better characteristics for high density recording has been carried out.
Well known methods for preparing ferromagnetic metal powders include the following:
(1) a method which comprises thermally decomposing an organic acid salt of ferromagnetic metal and reducing it with a reducing gas, as disclosed in U.S. Pat. Nos. 3,574,683, 3,574,685, 3,855,016, 3,843,349 and 3,892,673;
(2) a method which comprises reducing an acicular oxyhydroxide, which can contain one or more other metals, or an acicular iron oxide obtained from the oxyhydroxide (iron oxide reduction method), as disclosed in Japanese Patent Application (OPI) No. 97738/74 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), U.S. Pat. Nos. 3,607,219 and 3,702,270;
(3) a method which comprises evaporating a ferromagnetic metal in an inactive gas under low pressures (low pressure evaporation method), as disclosed in Japanese Patent Publication Nos. 15320/74 and 18160/74;
(4) a method which comprises thermally decomposing a metal carbonyl compound, as disclosed in U.S. Pat. Nos. 3,172,776, 3,200,007 and 3,262,812;
(5) a method which comprises electrically separating a ferromagnetic metal powder using a mercury cathode and then separating the resulting metal powder from the mercury, as disclosed in U.S. Pat. Nos. 3,156,650 and 3,262,812;
(6) a method which comprises reducing a ferromagnetic metal salt by adding a reducing agent to a solution of the ferromagnetic metal salt (borohydride method), as disclosed in U.S. Pat. Nos. 3,669,643, 3,672,867 and 3,726,664.
Of these methods, methods (2), (3) and (6) are conventional due to their practicality and the characteristics of the medium produced, and method (2) is most practical from the economical standpoint.
A ferromagnetic metal powder has coercivity (H.sub.c) and saturation magnetization (.sigma..sub.s) more than conventional ferromagnetic powders such as iron dioxide or chromium dioxide, and, therefore, is expected to be useful as a ferromagnetic powder for high density recording medium. In particular, recent trends in video tape recorders (VTR) are to reduce their size so that the recorders can be integrated with a video camera and get to improve image and sound qualities over those of VHS/.beta. systems presently used. However, there are many problems when the ferromagnetic metal powders are employed for high density recording. One problem is that metal particles coagulate with each other due to the large saturation magnetization, and, hence, it is difficult to obtain a magnetic layer having better surface property where they are coated on a non-magnetic support. Accordingly, there are problems of obtaining sufficient output due to the spacing loss generated between the magnetic head and recording tape. Further, more noise is present and a high S/N cannot be obtained. Another problem is that the metal particles are easily oxidized or hydroxidized due to the use of metal powder, and, hence, the magnetic properties of the resulting metal powder deteriorate.
Various methods have been proposed to improve these problems. However, it is difficult to satisfy the characteristics of magnetic recording medium and the stability of ferromagnetic metal powder, simultaneously. For example, it is proposed that surfaces of metal powders are gradually oxidized to improve the stability of the metal powders. An oxidized layer is formed around the powders to guarantee the stability. However, this causes the saturation magnetization (.sigma..sub.s) to be lowered and a magnetic recording medium having high S/N ratio cannot be obtained. The stability is worse as the metal particles are smaller. The stability can be increased by mixing the metal powders with a binder to make a magnetic layer, but is is not still sufficient.